Method for manufacturing molded articles of polypivalolactone resin which are superior in rigidity and toughness

ABSTRACT

A method for manufacturing molded articles of polypivalolactone resin which have highly increased mechanical properties, especially in toughness, said cell comprising subjecting highly crystalline molded plastic articles of polypivalolactone resin having an intrinsic viscosity ( Eta ) in the range of 1.5-4 to heat treatment under the treatment conditions consisting of temperature and duration which are both in ranges specified.

United States Patent Tohru Kitazawa Osaka-shi;

Masao Onga, Kobe-shi, both of Japan 702,642

Feb. 2, 1968 Dec. 7, 1971 Kanegaiuchi Boseki Kabushiki Kaisha Tokyo, Japan Feb. 10, 1967 Japan Inventors Appl. No. Filed Patented Assignee Priority METHOD FOR MANUFACTURING MOLDED ARTICLES 0F POLYPIVALOLACTONE RESIN WHICH ARE SUPERIOR IN RIGIDITY AND TOUGlI-INESS 10 Claims, 3 Drawing Figs.

U.S. Cl 264/176 R,

260/783 R, 264/235 Int. CI 1329i 3/08 DURATION OF HEAT TREATMENT (hr) mo :40 me Primary Examiner Robert F. White Assismm Examiner-Gene Auville Attorney-W0odhams, Blanchard and Flynn ABSTRACT: A method for manufacturing molded articles of polypivalolactone resin which have highly increased mechanical properties, especially in toughness, said cell comprising subjecting highly crystalline molded plastic articles of polypivalolactone resin having an intrinsic viscosity [1;] in the range of 1.5-4 to heat treatment under the treatment conditions consisting of temperature and duratidn which are both in ranges specified.

w 410121 n .e

-owsm i034 HEAT TREATMENT TEMPERATURE (c) PATENTED DE 7 l9?! DURATION OF HEAT TREATMENT (hr) DURATION OF HEAT TREATMENT( hr) SHEET 1 [1F 2 m (4) y =-O.Ol2x+ ".5 FIG I y-O.45x+103.4

y.= -I.9x+24Z5 2* y/ 5) =-o.oo45 +1.:

I I 160 I80 200 220 240 HEAT TREATMENT TEMPERATURE (c) FIG. 2 f f (6,

i I l l I T l l i l T y=0.003x -1.14X +1059 L w); T

l 1 I40 I I 200 220 240 C) INVENTORJ fW/Fl/ ////,14Z/1l 4/4540 0fl 5/1 PATENTEU'UEB 7 l97| DURATION OF HEAT TREATMENT (hr) SHEET 2 BF 2 I20 I40 160 I80 200 220 240 HEAT TREATMENT TEMPERATURE (c) //v VEN TORS My? MHZ/WA METHOD FOR MANUFACTURING MOLDED ARTICLES OF POLYPIVALOLACTONE RESIN WHICH ARE SUPERIOR IN RIGIDITY AND 'IOUGI-INESS polypivalolactone resin, not having; undergone subsequent heat treatment, are sufi'lciently high in rigidity but are rather poor in toughness. On the basis of the data contained in this table, molded articles of polypivalolactone resin can be BACKGROUND OF THE INVENTION 5 termed briefly and adequately as being rigid and brittle 1. Field of the invention plastic articles." a

The present invention is concerned with a method f In order to enhance the impact resistance of molded plastic manufacturing molded articles of polypivalolactone resin articles in general, it ishecessary t arrange 50 that the applied which are of highly improved mechanical properties, espehtgh Impact energy absorbed some form Othercially in t h 2, D i i f h prior art to Theoretically, in the event that a certain high impact energy is Polypivalolactone resin is of a crystallinity which is so high pp to a molded F' articles in general thts energy is that the articles molded with polypivalolactone resin accordmmedlately ttahsmltted from one Surface thereof through ing to the injection molding techniques, compression molding Short Paths to the reverse face of the molded Flashc article. h i extrusion ldi techniques bl ldi and the molded plastic article is eventually destroyed. Theretechniques,or like molding techniques, are invariably superior fore, if the molded plastic article is constitutionally so arespecially in compressive strength and Rockwell hardness, of ranged that this pp Impact n gy is cffccuvcly mi g ed all the mechanical strengths. Also, these molded plastic artiand dispersed Within the internal structural System and also cles are superior also in heat-resistivity and in the resistivity to that he pplied impact energy is transmitted through long chemicals, owing to the nature of polypivalolactone resin that Paths to the reverse facet then the Impact gy Wlll be nit has a highly increased crystallinity. While molded articles of hated during its Course of transmission 30 that the molded polypivalolactone resin may be considered uitable f use as plastic articles eventually will gain an ability and force to resist industrial materials in view of the aforesaid desirable properthe impact Which has been pp lherelo- A a m 11 f r ties, it is important for those industrial materials of this type to reforming the te na fine s ructures of molded plastic artibe provided with both of the following principal properties of Cles which eet this purpose of enhancing their impact retwo categories which are: short term properties (which mean sistance, there have been practiced a method which involves ordinary dynamic properties) and long term properties (which the inclusion of an appropriate amount of soft rubberlike submeans dynamic properties which are displayed when stance, such as polybutadiene, in the hard plastics and also a deformed for an extended period of time or when loaded with method h h nvolves the step of copolymerizing the hard a weight for an extended period of time). In other words, these plastics n Such a 80ft r lik anceindustrial materials must be highly rigid as a whole body and at Plastics other than polypivalolactone resin which have the same time they must have a highly increased toughness. physical properties similar to those of the rigid and brittle This toughness can be evaluated from the impact resistance polypivalolactone resin include, for example, polystyrene and also from the degree of elongation of the molded plastic resin and polymethyl methacrylate resin. articles, both of which serve as indices of toughness. With the In case polystyrene resin is used, there is a known successful foregoing in mind, various properties of the molded articles of method of enhancing both the rigidity and the impact repolypivalolactone resin (not heat-treated) are compared with sistance. This enhancement is effected by preparing a threethese of other molded plastic articles in the following table 1. element copolymer of acrylonitrile-butadiene styrene. This IXEIIE 1 u Impact resistance Tern era- Tensile Compressive (of Izod Deformature C.) strength Elongation strength Hardness notch) kg. tion 4.6 18.6 kg./ (kg/cm!) (percent) (kg/cm!) (Rockwell) crn./crn. kg./crn. cm.

Polypivalolaetone 400-470 8-11 silo-1,020 M (80-97) 4. 9-5. 0 148-156 Polyac 600-700 26-65 1,000-1, 100 M -05 110 ABS MHB 510-530 4. 3-10 800-820 R -120 Polycarbonatenm. 560-670 6-23 770 M 60-70 Nylonfi 650-840 25-70 470-880 R-100-120 Polypropylene 300-420 200-700 590-700 Rs5-110 Methyl p01yrnethactylate 400-770 2-10 840-1, 300 M -105 Polystyrene 350-630 1-2.5 BOO-1,100 Mes-s0 AS 4 670-840 1. 5-3. 5 980-1, 200 M 80-90 .1

1. 325-340 12-17 420 R 80-120 High density polyethylene 250-300 15-100 700 1) 60-70 6-30 60-85 Low density polyethylene v 130-200 -4550 300-500 D 40-45 Unbreakable 40-50 Hard vinyl chloride 350-600 2-4 550-900 D 80-00 1.7-8.6 s Soft vinyl chloride -250 200-250 60-120 1 Can vary with the plasticizer used.

In the above table I, the tensile strength was measured on represents another example where a rubberlike component, Dumbbell type test pieces at the pull rate of 10 mm./min. Acwhich is mixed as the polymer blend in the copolymer, plays cording to D-638, ASTM, using an almighty tester manufacthe role of a resistor to the applied impact energy under the tured by K. K. Shimazu Seisakusho. principle as has been described above.

Compressive strength was measured according to D-785, 60 Also, in molded polypropylene resin articles, for example, it ASTM on square bars for impact test of l X one-half onehas been known to enhance the impact resistance of this fourth inch in size. molded plastic article by the admixture therein of a high pres- Hardness was measured according to D-785, ASTM on sure polyethylene or an ethylene-vinyl acetate copolymer. square bars for impact test of5 X one-halfX one-fourth (inch) For the purpose of improving the toughness of in size. 65 polypivalolactone resin, there was made an extensive research Impact resistance was measured, according to Izod method b t t Fl of p resistance which is imparted to f test f impact resistance D456 ASTM on square this polypivalolactone resin by blending therewith, according type test pieces prepared by injection molding process and to a know" techhologlcal Concept, Various P y Such as having a mechanically produced V-shapc notch of one-tenth 7O potybutadlehet g Pressure p y hy n hy n nyl inch in depth.

Heat resistance was evaluated and the result is expressed by the resistance to heat deformation, or in other words, it is indicated by the deformation temperature according to D-648, ASTM.

As will be noted clearly from table 1, molded articles of 75 acetate copolymer, oxidized polyethylene and ABS resin. However, the result was that these polymers which were blended in polypivalolactone resin were generally poor in their cosolubility with the latter, and therefore, no satisfactory effects were obtained in view of the high crystallinity of polypivalolactone resin.

Also, depending on the type of polymer blended, the molded polypivalolactone resin articles exhibited an enhanced impact resistance which, however, was accompanied by a substantial reduction in the desirable properties which are peculiar to polypivalolactone resin. Furthermore, a study was conducted by us, on the degree of impact resistance of molded articles of polypivalolactone resin which were produced by copolymerization of polypivalolactone resin and ,B-propiolactone homologues and which were not given subsequent heat treatment. The result was that there were obtained no sufficiently satisfactory molded plastic articles having both a high degree of rigidity and a high degree of toughness.

SUMMARY OF THE INVENTION It is, therefore, the object of the present invention to provide molded articles of polypivalolactone resin which are less brittle and which are of highly increased toughness.

In order to solve the foregoing problems of molded articles of polypivalolactone resin of the prior art, a basic study of the physical properties as well as the structure of polypivalolactone resin was conducted again and also an extensive research on the relationship between the molding conditions and the fine structure of the molded articles was conducted by us. It was found from this study and research that the impact resistance and the tensile elongation of the molded plastic articles could be markedly improved simply by subjecting the molded articles of a single-component polypivalolactone resin to heat treatment under specific treatment conditions, without the need of blending the polypivalolactone resin with any other polymers and without resorting the process ofsubjecting the polypivalolactone resin to copolymerization with any other polymers.

Hereunder are given the results obtained from subjecting molded articles of polypivalolactone resin shown in table I to dry heat treatment at 200C. for L5 hours.

It will be clearly understood from table 2 that molded articles of polypivalolactone resin which have undergone a heat treatment exhibit a marked improvement in their toughness, or in other words, a marked improvement in both the impact resistance and elongation which are the object of the present invention. Although this improvement in toughness and the increase in elongation are accompanied by a slight reduction in the tensile strength and the hardness of the molded plastics, such reduction practically is negligible and should cause no problem whatsoever in the physical as well as the chemical properties of the molded plastics.

it was noted from the research that molded articles of polypivalolactone resin which did not undergo a heat treatment showed a deformation temperature which was in the range of I48 -l56 C., and th at those molded plastics which did undergo a heat-treatment showed deformation temperatures above this range. Since polyethylene glycol, which was the heat medium used in the measurement in the aforesaid study and research was such that it could not be used at a temperature above 160 C., no precise values were obtained. In view of the fact, however, that the amount of the deformation of the heat-treated molded articles did not mark 0.0l inch even when they were subjected to the temperature of 160 C.,

it can be safely said that the deformation temperature of the molded articles of polypivalolactone resin which have undergone heat treatment would never be less than that of those molded articles of polypivalolactone resin which have not undergone heat treatment.

Molded articles of polypivalolactone resin having undergone heat treatment also showed extremely satisfactory results in the impact fatigue test, and this satisfactory resistance to impact fatigue constitutes one of the important properties of molded plastic materials intended for use industrially. More specifically, molded articles of polypivalolactone resin not having undergone heat treatment are brittle and are easily broken. According to the experiment conducted by us, those molded plastic articles which have been given a heat treatment showed an impact fatigue resistance which was improved to about I00 times that of untreated molded plastics. As will be noted from the aforesaid finding, the improvement in the toughness of molded articles of polypivalolactone resin which is effected from subjecting such molded plastic articles to heat treatment is something which is so outstanding that it is not consistent with the widely accepted conception of thermal behavior of ordinary polymers. That is to say, according to the accepted idea, highly crystalline polymers, when subjected to a heat treatment, would rather exhibit reduced impact resistance and elongation. For this reason, the aforesaid marked effect or the marked improvement in toughness which is acquired by molded articles of polypivalolactone resin when subjected to a heat treatment is something which should be termed as being really an unexpected and surprising phenomenon.

The aforesaid fact will be easily understood by comparing the thermal properties of, for example, molded articles of polyacetal resinwhich represent a typical industrial material of this kind as described previously-with the thermal properties of molded articles of polypivalolactone resin.

The following description will refer to Celcon resin, a product of Celanese Corporation of America, as a typical polyacetal resin. It is reported that the molded articles of Celcon resin, when subjected to a heat treatment at 1 16 C. for a period up to 2000 hours, do not exhibit any appreciable change in the tensile impact strength or lzod impact resistance, whereas the tensile strength of the molded plastic articles increases with the duration of the heat treatment given. The molded plastic articles of this type have a melting point (herein it means the temperature at which the crystals are melted) of C. and a flow temperature of 174 C. The temperature at which deformation of the molded plastic articles takes place at the end of 30 minutes of being left in quietude without being loaded with a weight, or in other words, the deformation temperature of this molded plastic articles, is in the range of l49-l 60 C. The aforesaid various kinds of temperatures of molded articles of polyacetal resin lie in ranges which are all considerably lower than the ranges of corresponding temperatures of molded articles of polypivalolactone resin.

Furthermore, in case it is intended to increase the dimensional stability of those molded plastic articles which require precision in various respects among all industrial materials, this stabilization has been often effected by subjecting the molded plastic articles to heat treatment. Such a stabilizing treatment which was given molded plastic articles of the prior art has been intended for the removal of the residual strains of the internal structure of the molded plastic articles, and the molded plastic articles have been given this stabilizing treatment only to prevent the occurrence of the so-called stresscracks which were caused either by the mechanical forces applied to or by the solvents used, or to enhance the dimensional stability by eliminating warping of the molded plastic articles. Therefore, such changes in the physical properties resulting from the heat treatment as are seen in the molded articles of polypivalolactone resin of the present invention are something which is really amazing and are completely unforeseen effects which are due to the substantial and drastic reformation of the internal fine structure of the molded plastic articles. In connection with the heat treatment of this polymer, there has been described in the Japanese Pat. Publication No. 9810/1966 a method for improving the work recoverability and the tension recoverability of polypivalolactone fibers by subjecting the latter to a heat treatment. The invention described in said publication relates to a polypivalolactone fiber and a manufacturing method therefor, which is intended fur use in carpets, pillows, sleeping bags, sheets, cushions, stuffing materials, reinforcing materials and so forth and wherein the numerical values to be assumed by the specific basic structural elements (which are molecular weight, degree of orientation, a-ratio, apparent width of the a-crystallite and radical breadth of the crystallite) of the fiber are defined.

The present invention is completely different from that described in said publication in the object of manufacture, use, state and structure of the products. Besides, the present invention does not satisfy the basic structural elements defined in said publication, and this is very clearly demonstrated when, for example, the method for manufacturing the molded plastic articles of the present invention is compared with the method of manufacturing said fibers, or when, for example, the degree of orientation due to the difference in the manufacturing method is taken into account. Furthermore, the mere comparison of both the structure and the values of the imparted physical properties of the fiber described in said publication with the structure and the values of the physical properties of the molded plastic articles of the present invention will readily demonstrate the differences between the present invention and the invention described in said publication.

The relationship between the internal structure which constitutes the tough molded articles of polypivalolactone resin of the present invention and the physical properties thereof will .hereunder be briefly discussed. Assuming, in general, that the attractive force exerted between the chain molecules represents a "combination" of molecules in its broad sense, this polymer consists ofa sort of network which is formed by molecules having various different degrees of attractive force.

By subjecting this molded plastic to heat treatment, there are formed and also there will be a growth of firm molecular combinations which are more securely united to each other of all the molecular combinations constituting the molded plastic article, and at the same time, there are formed relatively soft portions in the molded plastic where the segmental motion of molecular chains located between the aforesaid combinations can take place with relative freedom. The toughness which is imparted to the molded articles of polypivalolactone resin in accordance with the present invention is based on such relatively obvious two-phase structure of the polymer which results from the heat treatment. lt has been elucidated by us, that the changes which occur in the physical properties of the molded plastic articles of the present invention due to heattreatment are closely related to, especially, the amorphous areas whichare represented by the aforesaid latter soft portions.

It should be understood further that, in view of the object of the present invention being to achieve improvement of the toughness, which may be more specifically termed as being the impact resistance and the tensile elongability, of the molded plastic articles which may be produced according to various molding techniques such as injection molding, compression molding, blow molding and extrusion molding, the ranges of heat treatment conditions which are applied to the molded plastic articles of the present inventibri differnaturaTy and essentially from the ranges which are applied to the fiber described in the aforesaid publication.

The following description will next be directed to the details of the result of the research undertaken by us. on the correlation between the molding conditions, the heat treatment conditions-both of which are associated with the manufacture of the molded plastics-and the properties of the molded plastics as the final products.

In the research of the molding conditions applicable to, for example, injection molding, we, measured the various properties of the molded plastics under various combinations of molding conditions which range widely in such a way that the injection pressure ranges from 300 kgJcm. to l kg./cm. the metal mold temperature ranges from 20 to C., the cylinder temperature ranges from 245 to 320 C. and the cycle ranges from 20 seconds to 3 minutes (20 seconds to 3 minutes for dumbbell test pieces and 30 seconds to 3 minutes for square bars for impact test). Furthermore, various properties of the molded plastics which are acquired after heat treatment were observed by subjecting the molded plastic articles to heat treatment at temperatures ranging from 100 to 230 C. for periods of time lasting up to 10 hours.

Description will be made first on the molded plastic articles which have not been given heat treatment.

Optimum injection pressure varies with the molecular weight of the polymers used. Intrinsic viscosity [1 was employed to serve as the measure for differentiating the molecular weights. This [1 was determined on a mixed solvent consisting of six parts by weight of phenol and four parts by weight of orthochlorophenol and held at 30 C. As the result of the aforesaid molding experiment, it was found that the appropriate range of moldability was in the range of 400 to 900 kg./cm. (molding pressure) for plastics having an [1;] of 2.0 or less; 600 to 1000 kg./cm. for plastics having an [1 of 2.0-3.0; and 800 to 1100 kg./cm. for plastics having an [17] of three or more. The range of [17] which is appropriate for injection molding was L5 to 4, preferably 2 to 3.5. It was noted from the experiment conducted by us, that, in case the injection pressure was exceedingly great, there could occur cracks in the molded plastic articles, whereas in case the injection pressure was extremely small, there would occur lack offilling of the plastic in the metal mold, and as a result, it often happened that no satisfactory molded plastic articles were obtained in each of these instances. The metal mold temperature did not have a marked effect-on the properties of the molded plastic articles, and therefore, the appropriate range of this temperature can be selected as desired. ln practice, however, the selection of the metal mold temperature ought to be de- ,cided in association with the conditions of the subsequent heat treatment. While the relationship between the metal mold temperature and the heat-treatment temperature will be discussed later, it may be said that such selection can be done as desired depending on the internal structure of the molded plastic article which is determined from the aspect of the use of the molded plastic articles. Speaking generally with respect only to the metal mold temperature itself, it can be said that higher moldability is obtained from lower metal mold tem perature, whereas the properties of the final molded plastic article increases with an increase in the metal mold temperature.

The cylinder temperature was such that satisfactory moldability as well as satisfactory properties of the molded plastic articles were obtained from temperatures in the range of from 250 to 290 C.

The relationship between various molding conditions and the degree of their individual influence on the ability of the molded plastic articles which have not been given a heat treatment was studied. It was found that the most influential factor of all the molding conditions was the injection pressure, and that the metal mold temperature came next.

With respect to the cylinder temperature, no marked effect on the properties of the molded plastic article was observed. The degree of influence which is exerted by the cylinder temperature on molded polypivalolactone articles should differ somewhat from that which is exerted on ordinary polymers, but such a difference can be explained by the excellent flow property of the polypivalolactone resins. Judging from the increased properties of the molded plastic article having undergone a heat treatment, the metal mold temperature has as overwhelming influence on the properties of the molded plastic article. The lowest heat-treatment temperature (T) which is required for imparting toughness to the molded plastic articles and the metal mold temperature (Tm) at the time of the injection molding are related to each other in such a way as is indicated by the following equation (1):

In other words, the effect of the heat treatment develops at a temperature of about 140 C. at the lowest where the metal mold temperature is 30 C., and at a temperature of about 130 C. at the lowest where the metal mold temperature is 150 C.

' The degree of the effect of the heat treatment depends also on the metal mold temperature, and greater effect is exhibited by those molded plastics which have been produced according to the injection molding techniques at higher metal mold temperatures. In the event, however, that a molded plastic article not heat-treated yet was subjected to a heat treatment at, for example, 180 C. for 1 hour, the impact resistance of this treated molded plastic at Tm=30 C. was 4,48 kg.cm./cm. (in contrast to 3.80 kg.cm./cm.'* of the molded plastic article not heat-treated); at Tm=80 C., it was 5.46 kg.cm./cm. (as against 4.20 kg.cm/cm. of the molded plastic article not heattreated); and at Tm =l50 C., it was 8.66 kg.cm./cm. (in contrast to 4.90 kg.cm./cm. of the molded plastic article not heat-treated).

Ordinarily, the preferred metal mold temperature is lower than the deformation temperature of the polymer used. Accordingly, the preferred metal mold temperature for molding polypivalolactone resin is 160 C. or lower. While the dimensional stability of the molded articles of polypivalolactone resin increases with an increase in the temperature of the metal mold used, high metal mold temperatures not only brings forth a loss of economy due to the prolonged cycle, but also is economically disadvantageous from such point of view as the cost of electric heating. On the other hand, however, lower metal mold temperatures will result in a reduced properties of the molded plastic articles as compared with the properties resulting from high metal mold temperatures. Furthermore, different effects are produced on the properties of the molded plastic articles from different heat treatment conditions. With respect to the aforesaid test pieces of the molded article of polypivalolactone resin, a maximum effect of the heat-treatment on these test pieces was noted when the heat treatment temperature was 200 C., at which occasion the impact resistance showed a value which was 10.4 kg.cm./cm. As has been described above, the impact resistance of molded articles of polypivalolactone resin increases with an increase in the heat treatment temperature up to 200 C. With this 200 C. being the peak, the impact resistance of the molded plastics drops when the latter are subjected to heat treatment at temperatures above this level. This drop in the impact resistance noted from heat treatment conducted at temperatures above 200 C. is considered to be due to the participation of thermal degradation of the polymers used. More specifically, when a molded article of polypivalolactone resin is subjected to a heat treatment at a temperature above 200 C., at, for example 210 C. or 220 C., there will be noted coloring of the molded plastic article even from a heat treatment lasting for a short period of time. The degree of the effect, due to the heat treatment time lasting in excess of l hour, on the properties of the molded plastic article, is not markedly great as compared with the effect caused by the heat treatment temperature. However, the degree of reduction in the properties of the molded plastic article when the latter is subjected to heat treatment at a temperature above 200 C.at which time thermal degradation is considered to associate with this reduction in the properties increases to a considerable extent in proportion to the length of the heat treatment given.

Similar effects from heat treatment as those exerted on the aforesaid impact resistance also occur on the tensile elongation of the molded plastic articles. In contrast to the elongation of the molded plastic articles which have not been given a heat treatment i.e 9 percent, the elongation of those molded plastic articles which are subjected to a heat treatment at about 210 C. will increase up to the maximum of 60 percent. In other words, in case a molded plastic article is subjected to a heat treatment, its impact resistance increases twice as much or more of the level observed before such heat treatment was given, while the elongation of the molded plastic article improves five times as much or more of the value noted prior to such heat treatment.

The uppermost limit of the heat treatment temperature is 230 C. Temperatures beyond that level are consistent with the softening range of the molded plastic articles, and cause an even further reduction in the properties of the molded plastic articles beyond the level of the properties noted on the molded plastic articles which are not heat-treated. It is added here that the melting point of the molded articles of a singlecomponent polypivalolactone resin is 238 C.

In general, the optimum result is obtained from heat treatment which is conducted within a temperature range of l80-200 C. With respect to the duration of heat treatment, it should, of course, vary depending on the temperature employed. Broadly speaking, however, a heat treatment lasting not longer than 6 hours will insure satisfactory operation without the fear for the occurrence of coloring in the molded plastic articles.

While no strict minimum length of heat treatment can be set forth, there are instances where both the impact resistance and the elongation show an improvement due to a heat treatment lasting for only 2 to 3 minutes. However, a heat treatment time lasting 30 minutes or more is preferred, in which case a very marked desirable effect is insured.

The heat medium which is employed in the heat treatment of the present invention is not restricted specifically. So long as it is an inactive heat medium which does not directly react chemically with polypivalolactone resin, any desired heat medium such as heated air, hot water, heated aqueous vapor, Wood's alloys and oily materials, can be used.

Description has been made in detail on the correlation between the molding conditions, the heat treatment conditions and the properties of the molded plastic articles of the type produced according to the injection molding techniques. This correlation essentially holds true for those molded plastic articles produced by the molding techniques other than said injection molding techniques. Hereunder will be described those points which constitute problems in the molding of plastics which is effected according to the molding techniques other than the injection molding techniques.

in the case of extrusion molding, its workability is, in general, better with plastics having greater molecular weight. Since this extrusion molding requires a somewhat high back pressure and also uniform melting of the plastic to be extruded, the extruding apparatus per se is required to be such that can satisfy very severe mechanical conditions as compared to the apparatuses used in other molding techniques. In view of the fact that the melt-viscosity of polypivalolactone resin, or in other words, its flow property when melted, depends greatly on both the temperature and the pressure applied to as compared with other ordinary polymers, the workability of molding of this resin is better than with other plastics. in extrusion molding, technological caution should be directed to the following points: the rate of pulling which is applied to the polypivalolactone resin after it has been discharged from the die; the distance between the die and the chilled rollers; and the balance between the temperature of the pulling rollers and the extrusion pressure. It should be noted further that the employment of a temperature above 280 C. would often cause coloring and decomposition of the polymer, so that it is important to place the temperature under articularly severe control. These technological elements will constitute important factors which determine the value, as a commodity, of the molded plastic product which may be, for example, a sheet, the quality of which will be evaluated by, for

example, the evenness or uneveness of the width and the thickness of the sheet; the absence or the presence of bubbles in the sheet; transparency or coloring of the sheet; and the quality of the finish of the faces of the sheet.

While individual molding conditions have to be selected adequately for each extrusion molding by taking into account the molecular weight of the polypivalolactone resin used, the specification of the molding apparatus and the purpose for which the product is used, there are the general molding conditions which are applied, in common, to all instances of extrusion molding. These conditions are as follows. First, with respect to the temperatures employed in the cylinder barrel section, there are the following ranges. Hopper section: 220-250 C., Cylinder: 260-280 C., Adapter: 260280 C., and Die: 250270 C., are preferred. Generally speaking, it is advantageous to carry out extrusion-molding of polypivalolactone resin in such a way that temperatures on the higher side within the aforesaid ranges be applied to polymers having a relatively high molecular weight, whereas temperatures on the lower side within said limits be employed for the polymers having a relatively low molecular weight.

The molded plastic articles thus produced are reformed into molded plastic articles having improved toughness by being subjected to a heat treatment which is conducted in a manner similar to that described in connection with the molded plastic products obtained according to the injection molding techniques.

Description will next be directed to the products obtained according to the compression molding techniques. The degree of the initial compression which is applied to the polymer in the stage immediately after the latter has been melted preferably is such that it is sufficient for only lightly pressing the metal mold against the contents so as to make the polymer to generally conform to the shape of the metal mold. With respect to the subsequent step of compression which is carried out in a cold press, it should be noted that a satisfactory molded plastic article can be obtained by the application ofa relatively high pressure to the metal mold and accordingly to the polymer contained therein while effectively utilizing the softening range of the polymer during the course of the cooling process. Animportant aspect of the compression molding lies in the manner in which the aforesaid latter steps are performed, or in other words, a successful compression molding is accomplished depending on the manner in which a pressure is applied to the cold press. Attention should be paid to the fact that polypivalolactone resin is of a high crystallinity and also that its softening range is narrow.

Description will next be directed to the molded plastic articles produced by the blow molding techniques. This blow molding is not particularly difficult technologically as compared with other molding techniques. Since the strain recovery of polypivalolactone resin at the point immediately after it is discharged from the die is rather intensive, there is no need of particularly taking the clearance of the die into account. Rather than that, attention should be paid to the high crystallinity of polypivalolactone resin. The exercise of this attention is necessary especially in the molding of molded plastic articles having a considerable thickness.

Molded plastic articles obtained according to the compression molding techniques and the blow molding techniques are also reformed into molded plastic articles which are satisfactorily rigid and tough and which have markedly improved impact resistance, elongation and fatigue resistance by subjecting the molded plastic articles to a heat treatment in a manner similar to that described above in connection with the molded plastic articles obtained by other molding techniques.

As has been stated above, we, have succeeded in manufacturing rigid and tough molded plastic articles without any appreciable loss of various mechanical properties which are possessed by polypivalolactone resin, based on the finding that molded articles of polypivalolactone resin are reformed, by being subjected to heat treatment, into useful industrial materials.

In actual heat treatment, it is necessary to set forth heat treatment conditions so as to adequately meet the history of llllll the molding conditions under which the plastics have been molded. It is to be noted, however, that irrespective of the conditions under which plastics are molded, it is possible to manufacture molded articles of polypivalolactone resin which are highly rigid and tough, by performing the heat treatment within the range of conditions which will hereunder be defined.

BRIEF DESCRIPTION OF THE DRAWING FIGS. ll, 2 and 3 are diagrams defining the optimum range of the heat treatment conditions which are applied to in carrying out the heat treatment of molded articles of polypivalolactone resin according to the method of the present invention, wherein FIG. 1 indicates the Range A which represents the necessary minimum conditions of the heat treatment; FIG. 2 indicates the Range B which represents appropriate conditions of the heat treatment; and FIG. 3 indicates the Range C which represents further desirable conditions of the heat treatment.

By summarizing the aforesaid result of the research undertaken by us, the treatment conditions according to the method of the present invention are strictly defined as follows. That is to say, the heat treatment is performed by selecting the temperatures and the heat treatment time (meaning the duration of the heat treatment) so that both lie within the Range A of FIG. 1, i.e. the area of oblique lines surrounded by the rectilinear lines which are indicated by the equation (2), (3), (4) and (5) which are as given below:

wherein:

x represents the heat treatment temperature C. and

y represents the duration of the heat treatment (hr.)

It is to be noted that the equation (3) and the equation (5) both represent the minimum treatment conditions which are necessary for imparting toughness to the molded plastic articles, wherein the former or the equation (3) defines mainly the heat treatment temperature, while the latter or the equation (5) defines the duration of the heat treatment. Also, the equation (2) and the equation (4) indicate the boundaries of the heat treatment conditions beyond which it is either actually difficult to perform the treatment or undesirable from the aspect of workability of the operation and/or the aspect of properties of the final products.

In order to obtain excellent molded plastic articles, however, it is desirable to select the heat treatment conditions so that they lie within the Range B of FIG. 2, i.e. the area of the oblique lines surrounded by the curves which are indicated by the equations (4), (5) (6) and (7), respectively, wherein the latter two equations are given as follows:

It is further desirable to perform the heat treatment by selecting the heat treatment conditions so as to fall within the Range C of FIG. 3, namely, the area of the oblique lines surrounded by the curves which are indicated by the undermentioned equations (8), (9), l0) and l l By doing so, it becomes possible to manufacture molded plastic articles which are extremely good in both rigidity and' plastic articles consisting of only polypivalolactone resin and containing no additive such as heat stabilizer can be easily reformed into molded plastic articles which are equipped with sufficiently good properties. In case, however, the heat treatment is conducted under conditions lying outside the curve which is indicated by the equation l), there can be instances where coloring of the molded plastic articles occur at the time of the heat treatment.

In the appended examples of the present invention are given detailed description of the significance of the aforesaid ranges of the heat treatment conditions.

Polypivalolactone resin which is used in the method of the present invention means a linear condensation polymer consisting substantially of recurring ester structure unites of the formula: 3

apparatus manufactured by Yamashiro Seiki K. K., both dumbbell type and square-bar-type test pieces were prepared.

Molding of these test pieces were performed under thev The molded plastic articles thus obtained were then subjected to dry heat treatment utilizing hot air held at 200 C. for 1.3 hours. The altered physical properties of the molded TABLE 3.*--MOLDING CONDITIONS AND PROPERTIES OF MOLDED PLASTIC ARTICLES Metal Compres- Cylinder mold Injection Tensile Elongasive Hardness Impact temp. temp. pressure strength tion strength (Rockwell) resistance 0.) C.) (kg/em?) (kgJcmJ) (percent) (kg/cm (kg.cm./cm.

Test piece No.:

1 245 160 1100 Poor molding 250 1200 Cracks developed in molded plastic 250 20 1100 400 11.0 850 86 3. 7 260 20 600 360 11. 0 720 80 3. 7 250 800 400 8. 0 840 86 3. 8 250 30 600 370 11. 0 720 82 3. 7 250 80 800 415 8.4 870 88 4. 2 250 80 600 390 11. O 770 83 3. 7 250 160 800 460 9. 3 000 86 4. 9 250 150 600 430 11. 0 870 82 3. 7 250 180 800 470 9. 6 1020 00 6. 1 250 180 600 450 11.0 050 86 3. 7 250 180 400 400 11. 0 900 86 3. 1 250 180 300 Luck of filling 270 30 800 400 8. 0 900 85 4. 2 270 30 400 380 9. 0 720 80 4.0 270 80 800 415 8. 1 910 85 4. 4 270 160 800 445 l). 0 930 87 4. 8 270 150 300 Lack of filling 290 80 800 410 0. 0 850 86 4. 3 290 150 800 450 10. 0 000 86 4. 8 290 150 300 Lack of filling 310 160 800 435 E). 0 850 85 4. 8 310 150 300 Lack of filling 320 150 800 Unmoldable due to intensive thermal decomposition and can be manufactured easily according to the method of polymerizing hydroxypivalic acid or its esters as is described in the specification of U.S. Pat. No. 2,658,055 or by the employment of the method of polymerizing pivalolactone as is described in the specification of the British Pat. No. 766,374. Besides these polypivalolactone resins described in these patents, copolymers which are prepared by subjecting such a polypivalolactone resin as is described in these patents to copolymerization with up to 25 percent by mol of other lactones such as ,B-propiolactone, a, a-diethylpropiolactone are also conveniently usable. Furthermore, mixtures prepared by blending such a polypivalolactone as has been described in said patents with other polymers which substantially do not affect the desirable inherent properties of the polypivalolactone resins are also suitable for the method of the present invention. It is needless to say that polypivalolactone resins containing ordinary additives such as dyestufis, pigments and stabilizers can be used also in the present invention.

The method of the present invention can be applied to molded plastic articles, molded plastic sheets, films, guts, filaments and like molded plastic products which are obtained according to the known molding techniques such as extrusion molding, injection molding, compression molding and blow molding.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1 Using polypivalolactone resin having an intrinsic viscosity [1;] of 2.88 and by the use ofa vertical type injection molding plastic articles after the heat treatment, are enumerated in the table 4 given below.

The method of measurement and the condition therefor for each item of measurements was pursuant to those described previously in the earlier part of this specification.

TABLE lr-PROIERTIES OF MOLDED PLASTIC ARTICLES AFTER HEAT TREATMENT (200 C. for 1.3 1101118) Compres- Impact Tensile Elongasive Hardness resistance Test strength tion strength (Rockwell (kg.cin./

piece (kg/cm!) (percent) (kg/cm?) M) rm!) 430 60 I050 82 ll. 9

In table 3, the molded plastic article of test piece No. l bore the formation ofseveral rows of linear marking which spoiled the external appearance, and besides, it had no luster at all.

The imparting of toughness to the molded plastic articles which is the object of the present invention was not accom plished on these test pieces even by varying the molding conditions in many ways. Only slight improvement in tensile strength and rigidity was noted where greater injection pressure was applied to and where higher metal mold temperature was employed. No obvious trend was noted in regard to elongation. However, in some instances, there were seen rather reverse behavior of elongation relative to rigidity where high injection pressure was applied to.

As shown in table 4, by subjecting these molded plastic articles to a heat treatment, there is obtained an extremely marked improvement in toughness, and as a result, the resulting molded plastic articles can be used as materials which are useful for industrial purposes.

EXAMPLE 2 Using a polypivalolactone resin having an intrinsic viscosity [1 of 3.0, molded plastic articles were prepared under the molding conditions consisting of a metal mold temperature of 150 C., a cylinder temperature of 250 C., an injection pressure of 800 kgJcm. 2 and a cycle of 1 minute for dumbbell test pieces and 2 minutes for square bars.

Description will hereunder be directed to the details of the various effects obtained from subjecting these test pieces to heat treatment under the respective treatment conditions lying within the ranges of A, B and C which have been described in the earlier part of this specification and also under treatment conditions outside these respective ranges. Table 5 given below contains the respective treatment conditions of the heat treatment given and their corresponding ranges.

Also, gross observations were made on the development of coloring by which the value of the molded plastic articles as commodity is evaluated in general.

TABLE 5.-THE

PROPERTIES TREATMENT The conditions of the heat treatment defined according to the present invention have been determined by taking into account those elements which are associated with the physical properties, external appearance and coloring of the molded plastic articles, and those elements related to the molding workability, and further those concerning economy. As has been stated in detail in the earlier part of this specification, the effect of imparting toughness is brought about by the adoption of the treatment conditions lying within the Range A. Desirably, by performing a heat treatment under the conditions lying within the Range B, and further desirably, by carrying out the treatment under the conditions lying within the Range C, it becomes possible to produce molded plastic articles having a highly increased commodity value.

EXAMPLE 3 Molded plastics were prepared with a polypivalolactone resin having an [1 l=2.6 and under molding conditions consist- 2O ing ofa cylinder temperature of 250 C., an injection pressure ways, the effect of the heat treatment on the test pieces was evaluated. In this example, measurement was taken on the effect of the heat treatment, i.e., the toughness, especially in regard to impact resistance and elongation which are shown in table 6.

TABLE 6.*RELAIIONSHIP BETWEEN HEAT TREATMENT CONDITIONS AND TOUGIINESS Heat treatment conditions Impact resistance Elongation Temp. C.) Duration (hr.) (kgtmjernfl) (percent) Not heat-treated 4. o 0. 3

.05 4. it (I. 3

DEFINED RANGES OF HEAT TREATMENT CONDITIONS AND THE OF MOLDED PLASTIC ARTICLES HAVING UNDERGONE HEAT Conditions of heat treatment Conipres- Hard- Tensile Elongasive ness Impact Temp. Duration strength tion strength (Roekresistance 0.) (hr.) Range (kg/em!) (percent) (kgJemfl) well M) (kg. cmJcmfi) Coloring Not treated 450 9. 3 900 86 4. 9 1 20 450 9. 3 900 86 4. 9 Nil. 140 0.3 ..d 450 9.3 900 86 4.9 Nil. 140 1.0 A 440 13 1,000 90 6. 4 Nil. 140 4 436 14 1, 040 89 6-6 Nil. 140 9 430 15 1, 09.3 88 7-0 Nil. 140 15 429 1,100 88 7. 0 Nil. 160 0. 2 o. 450 9 'i 900 86 4- 9 i 160 1 430 19 1,050 85 8.0 Nil. 160 4 424 21 1 100 84 8. 4 Nil. 160 9 420 25 1, 84 8. 7 Present. 15 400 20 1,105 80 8- 0 0- 0.1 450 3 d 900 86 4. 9 N l. 180 1 420 26 l, 050 82 9. 5 N 11. 180 4 411 2! 1,105 81 9.8 Nil. 180 9 405 35 1,154 80 10. 0 Present. 180 15 297 .27 1, 069 78 8. 9 o. 00 0 1 450 9 d 900 86 4. 9 Nil. 200 1 400 55 1,080 80 10.9 Nil. 200 4 .390 57 1, 089 77 11.0 Present. 200 9 A. 381 57 1, 095 75 11.1 Do. 200 15 Outse.. 363 50 1,051 71 10.5 0- 220 0.05 .do. 450 9. 3 900 86 4.0 Nil. 220 1 P80 48 1,030 78 10. 1 Present. 220 4 573 .39 060 77 8. 9 D0- 220 9 Outside 358 25 900 69 8. 0 D0- 230 0.01 .do. 450 9. 3 900 86 4.9 Nil. 230 0.05 A 450 10 910 84 5.1 Present. 230 1 Outside. 260 20 950 74 7- 0 Do.

l0 l0 l 3 0297 In table 6, the values of the test pieces Nos. 2, 3, 4 and 13 represent those which have undergone heat treatment under conditions outside the ranges defined by the present invention. From table 6, it will be understood that the toughness, the imparting of which is the object of the present invention, is marked improved by subjecting molded articles of polypivalolactone resin to a heat treatment. cl EXAMPLE 4 Using a polypivalolactone resin of ['fl]=2.8, test pieces of molded plastic articles were prepared under the same molding conditions as those in example 3. After subjecting them to a heat treatment, their impact fatigue resistances were measured. This measurement was performed by first mechanically forming a V-shape notch of 0.1 inch in depth in each test piece, and then giving impact to this test piece by dropping a weight of 246 gr. onto the test piece secured to a support at one end thereof from a height of 7 cm. measuring thereabove, by the use of an impact fatigue tester manufactured by K. K. Ueshima Seisakusho. The result of this test which was evaluated by the number of droppings of the weight till the tests piece was broken is shown in table 7.

As a reference, the result of similar tests conducted on control test pieces consisting of molded ABS (MV) resin similar in properties to polypivalolactone resin is given in this table.

TABLE 7 Relations between heat treatment conditions and fatigue By subjecting the molded plastic articles to a heat treatment under the heat treatment conditions defined according to the present invention, the impact fatigue resistance was markedly improved and the resulting molded plastic articles could thus be used as materials useful for industrial purposes.

EXAMPLE An important aspect of the operation which requires caution is the prevention of the occurrence of coloring in the molded plastic articles. Caution should be exercised when the heat treatment is performed in accordance with dry heat treatment and when this treatment is carried out at a high temperature or formany hours. In this example, the problem of coloring of the molded articles of polypivalolactone resin which are subjected to dry heat treatment is taken up, and the limits of both the temperature and the duration of the heat treatment till coloring occurred were sought. The result is shown in table 8.

The test pieces used in this example has an [n] of 303, and were prepared under the molding conditions consisting of a cylinder temperature of 250 C., an injection pressure of 800 kg./cm. a metal mold'temperature of 120 C. and a cycle of 1 minute for dumbbell type test pieces and 2 minutes for square-bar-type test pieces.

TABLE 8 Relations between heat treatment conditions and coloring Heat treatment conditions Coloring Temp. C.) Duration (hr.)

20 None 10 Present 150 5 None 4 None 180 5 Present I80 10 Present (marked) 200 1 None 200 2 Present 200 to Present (marked) 220 0.05 Present 220 1 Present (marked) From the result of table 8, it is known that the heat treatment conditions according to the present invention which satisfy the aforesaid equation (10) do not give rise to coloring of the molded plastic articles.

In practice, however, molded plastic articles usually do not consist of a single component of polypivalolactone resin alone. It is usual that these molded plastic articles contain stabilizers, pigments and/or other additives which are used to meet the purposes for which the molded plastic articles are used. Therefore, it is possible that the limitation which is given by said equation deviates to some extent without any actual harm on the molded plastic articles. Such a slight deviation can be exercised as desired so as to meet the purpose for which the molded plastic articles are used.

EXAMPLE 6 This example is intended to elucidate the relations of heat treatment conditions to hardness, bending strength and compressive strength. Using a polypivalolactone resin [1;]=2.7, molding was performed in a manner similar to that described in example 3. Heat treatment was conducted under the heat treatment conditions as shown in table 9.

TABLE 9 Conditions of heat treatment The properties of the molded plastic articles which have 70 been given a heat treatment is shown in table 10.

From table 10, it will be understood that other than a slight increase in the bending and compressive strengths which is resulted from the heat treatment performed under the preferred heat treatment conditions, there is noted no sub- 75 stantial effect of the heat treatment as a whole.

TABLE 10 Hardness Bending Compressive (Rockwell strength strength Test piece number M) (kg/0111. (kg/cm?) Not heat-treated as 480 910 5 EXAMPLE 7 ln molding a polypivalolactone resin, there naturally a range of intrinsic viscosity [1;] which is appropriate for the molding.

The appropriate range of grades of the intrinsic viscosity of polypivalolactone resin has been described in the earlier part of this specification. 1n the present example, however, test pieces having intrinsic viscosity [1;] of 1.1, 2.9 and 4.8, respectively, were used. The result of the observations of the molding of these test pieces under various molding conditions and of the subsequent heat treatment given these molded test pieces will be hereunder described.

First, the molding conditions and the evaluation of the external appearances of the molded plastic articles are shown in table 1 1.

described. It was found as the result of the experiment conducted by us, that in both the blow molding and the extrusion molding, the adoption of polypivalolactone resins having a higher intrinsic viscosity [1 brought forth satisfactory results in general, and that in the injection molding and the compression molding, the adoption of polypivaloactone resins having a lower intrinsic viscosity [1;] gave satisfactory results.

With respect to the result of the heat treatment, those plastic articles which have been molded under satisfactory molding conditions acquire a highly increased toughness due to the heat treatment given, as has been already described in this specification.

EXAMPLE 8 A polypivalolactone resin having an [1;] of 3.6 and mixed with 0.1 percent by weight of trioctadecyl phosphite and 0.5 percent by weight of dilauryl thiodiprop ionate was used. This mixture was subjected to compression molding by the use of an oil pressure-heat press manufactured by K. K. Shindo Kinzoku Kogyosho. By cooling this initial molded plastic article while being compressed in the cold press, a sheet-form molded plastic article having a thickness of 3 min. was obtained.

The molding conditions employed in this compression molding consisted of the temperature of 260 C. and the pressure of 200 kg./cm. The resulting molded plastic article was given a steam heat treatment at 180 C. for 1 hour. Dumbbell TABLE ll.-MOLDING CONDITIONS AND MOLDED PLASTIC ARTICLES HAVING VARIOUS VALUES OF (n) Injection Metal mold Cylinder pressure temp. temp. C.) (kg/em!) C.) 1.1 2.9 4.8

400 150 Lack of filling 1, 100 150 Poorly molded 200 150 Lack of filling 400 150 Satisfactory .1 Lack of filllng 600 150 .do setisfectoryntu Lack of filling. 800 150 do ..d0 Do. 200 150 Lack of filling 400 150 Satisfactory Lack of filling 600 150 .1. .(i0 Satisfact0ry Lack of filling. 800 Do. 1, 000 Do. 600 Do. 1,000 d0 D0.

600 150 Cracks and colorecL Cracks and colored. 1,000 150 .....(10 D0.

In table 1 l, the properties of those satisfactorily molded. plastic articles which have been given a heat treatment at 200 C. for l lhour are shown in table 12.

type test pieces were obtained by punching this sheet, and

they were evaluated as regards their properties. The result of this evaluation is shown in table 13.

TABLE 12.MOLDING CONDITIONS AND PROPERTIES OF MOLlDED PLltsTro Andrews rmvlNo UNI)ER GONE EEAT TREATMENT Metal Comprcs- Hard- Cylinder Injection mold Tensile Elonga sive ness Impact Bending temp. pressure temp. strength tlon strength (R0ck resistance strength C.) (kg/cm!) C.) (kg/em!) (percent) (kg/cm!) well M) (kg. cm./cm. (kgjcmfi) In the injection molding, its molding workability is such that TABLE 13 the appropriate range of [1 which has been described in the earlier part of this specification varies somewhat depending on the shape and the size of the gate, sprue and runner of the 70 Tensile Elongation Hardness Impact metal mold used. However, satisfactory molded plastic articles strength2 R k H M risistantie 2 are manufactured so long as the injection molding is per- (kg/cm) we formed by the use of a polypivalolactone resin having an intrinsic viscosity 1;] in the range as described in the earlier part 59 88 of this specification and under the molding conditions herein 75 The impact resistance shown in the above table l3 was measured on those test pieces of l27Xl2.7X3 (mm.) in size which were punched from said sheet for the exclusive use in this example.

EXAMPLE 9 A polypivalolactone resin of [17]=2.9 and containing 0.17 percent by weight of titanium dioxide as the delustering agent was used. The mixture was subjected to molding under the molding conditions consisting of the cylinder temperature of 250 C., the injection pressure of 1000 kg./cm.", the metal mold temperature of 180 C. and the cycle of 1 minute for dumbbell type test piece and 2 minutes for square-bar-type test pieces. The molded test pieces were then subjected to a heat treatment at 200 C. for 1 hour. The result of the evaluation of the properties of these test pieces is shown in table 14.

TABLE 14.PROPERTIES F MOLDED PLASTIC ARTICLES HAVING UNDERGONE HEAT TREATMENT Compres- Impact Tensile Elonsive Hardness resistance Bending strength gation strength (Rock- (kg.cn1./ strength (kg/cm?) (percent) (kg/cm?) Well M) cm?) (kg. 1cm!) EXAMPLE 10 TABLE 15.PROPERTIES OF MOLDED PLASTIC ARTICLES I MANUFACTURED ACCORDING TO COMPRESSION MOLDING TECHNIQUES Impact Tensile Elon- Hardness resistance strength gation (Rock- (of Izod notch) (kg. lam!) (percent) well M) (kg. cmJcmJ) Not heat-treated 204 3. 5 86 22. 1 Heat-treated... 195 18. 6 83 31. 3

in table 15, the impact resistance was measured with a UF Impact Tester manufactured by K. K. Ueshima Seisakusho, on test pieces of 40X10X4 (mm.) in size and having no notches which were prepared by processing said discs.

The hardness was measured, with a Rockwell hardness tester (type ARKA) manufactured by K. K. Ueshima Seisakusho, on the aforesaid test pieces.

The tensile strength and elongation were measured on films of 0.2 mm. in thickness which were prepared from said discs, and by the use ofa Tensilon manufactured by Toyo Sokki K. K., before and after the molded plastic articles were given a heat treatment.

As is clear from table 15, a marked effect of heat treatment is seen also in the molded plastic articles which are manufactured according to the compression molding techniques, and there is no loss, due to the heat treatment applied of the desirable properties such as rigidity and tensile strength of the polypivalolactone resin used.

EXAMPLE I l Using a polypivalolactone resin of [1 ]=4.0, and using an extruder of 40 mm. this polymer was supplied, via a hopper which was held at 240 C., to a cylinder which was held at 275 C., and therefrom the polymer was extruded, in sheet-form, from an open die. Thus, a sheet having a thickness of 10 mm. was manufactured. The sheet thus produced was then subjected to a heat treatment at l C. for 2 hours.

The result of the evaluation of the properties of the molded plastic articles is shown in table 16.

As is clear from table 16, satisfactory efiect is afforded by the heat treatment. It is noted that a marked increase in toughness is imparted to the molded plastic articles without impairing the desirable properties of the polypivalolactone resin used.

TABLE 16 Properties ofthe molded plastic articles manufactured according to the extrusion molding techniques Compressive Hardness Impact strength resistance (of Izod notch) (kg/cm!) (RuckwelLM) (kg.crn./cm.)

Not heat-treated I045 86 5.]

Heat-treated l 193 83 16.3

EXAMPLE 12 Plastic bottles of about 0.4 liter in capacity and having a small wall thickness were molded with a polypivalolactone resin of [1;]=4.0. Extrusion of this polymer was performed by the use of an extruder having 20 mm. diameter of screw and under the molding conditions consisting of a cylinder temperature in which the temperature on the hopper side was regulated to 230 C., and further consisting of the die temperature of 250 C., the metal mold temperature of 150 C. and the number of rotation of the screw of 35 rpm. The air pressure which was employed in expanding the parison was about 3.3 kg./cm. A dropping test from the height of 75 cm. was conducted on the bottles thus produced.

More specifically, when those molded plastic bottles of 400 cc. which were not heat-treated and which were filled with water were dropped from said height, the bottles were broken on one drop test.

In contrast to this, those molded plastic bottles having been given a heat treatment at l C. for l hour were broken for the first time on the 67th dropping test. Thus, a marked increase in toughness due to the heat treatment is noted also in molded plastic articles which are manufactured according to the blow molding techniques.

What is claimed is:

1. A method of manufacturing nonfiber-form extruded articles ofpolypivolalactone resin, which articles having highly increased rigidity and toughness, comprising the steps of extruding polypivalolactone resin having an intrinsic viscosity [1;] in the range of from 1.5 to 4.0 to produce nonfibrous extruded articles, wherein the temperature of the hopper section of the extruder is in the range of 220-250 C., the temperature of the extruder cylinder is in the range of 260-280 C., the temperature of the extruder adapter is in the range of 260-280 C. and the temperature of the extruder die is in the range of 250-270 C., and then subjecting the extruder articles to a heat treatment, by applying thereto a fluid heating medium which does not react with polypivalolactone resin, the heat treatment being carried out under treatment conditions consisting of temperature and duration which fall within the hatched area of FIG. 1, said hatched area being defined by the equations:

y-O.45x+l03.4 ;l.9rl-247.5 y;0.0045x+l .1 y 0.0 l 2x+l l.5 wherein:

x represents temperature C. and

y represents duration (hr.

2. A method according to claim 1, in which the treatment conditions fall within the hatched are of FIG. 2, said hatched area being defined by the equations:

wherein:

x represents temperature C.), and y represents duration (hr.

4. A method according to claim 1, wherein said polypivalolactone resin has an intrinsic viscosity [1;] in the range offrom 2.60 to 4.0.

5. A method according to claim 1, wherein said heat treatment medium is an inactive medium selected from the group consisting of heated air, hot water, heated aqueous vapor, Wood 5 alloys and heated oil materials.

6, A method according to claim 1, wherein said polypivalolactone resin contains a small amount of stabilizer.

7. a method according to claim 6, wherein said stabilizer is a substance selected from the group consisting of trioctadecyl phosphite and dilauryl thiodipropionate.

8. A method according to claim ll, wherein said polypivalolactone resin contains a small amount of titanium dioxide.

9. A method according to claim 1, wherein said molded plastic articles consist of a copolymer of polypivalolactone monomer and 25 mol percent of another lactone.

10. A method of claim 9, wherein said other lactone is one selected from the group consisting of B-propiolactone and a, a-diethylpropiolactone.

.. UNI-TED sTATEs PA'iENT CERTIFICATE 0E CORRECTION I, Patent 1 048 Dated December 1 I] 21] Invento.r(s)

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below":

Column 20, line 54; "having is corrected to read have---.

line-63; ext-ruder" iscorrected to read extruded Column 21, lines 5 -9; These lines are corrected to .read

' y? =0. '01-:g e-Ag4'2x- +4.8 -8.'4

O.O0..3 ;-;-'-1.; =14X 105.9

- Column 22 lih'efj; "oil" corrected-to read--oily--.

-"1in e 2O;--"of"' is corrected to read according to---.

Signed and sealed this 6th day of June 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM uscoMM-oc 60376-P69 [1.5. GOVERNMENT PRINTING UFFICE: I959 0"356'33 

2. A method according to claim 1, in which the treatment conditions fall within the hatched are of FIG. 2, said hatched area being defined by the equations: y -0.0045x+ 1.1y -0.012x+ 11.5y 0.01x2-4.42x+ 488.4y 0.003x2-1.14x+ 105.9 wherein: x represents temperature (* C.), and y represents duration (hr.).
 3. A method according to claim 1, in which the treatment conditions fall within the hatched area of FIG. 3, said hatched area being defined by the equations: y -0.033x+ 11y 0.017x+ 3.83y 0.002x2-0.92x+ 105.3y 0.003x2-1.2x+ 119.1 wherein: x represents temperature (* C.), and y represents duration (hr.).
 4. A method according to claim 1, wherein said polypivalolactone resin has an intrinsic viscosity ( in the range of from 2.60 to 4.0.
 5. A method according to claim 1, wherein said heat treatment medium is an inactive medium selected from the group consisting of heated air, hot water, heated aqueous vapor, Wood''s alloys and heated oily materials.
 6. A method according to claim 1, wherein said polypivalolactone resin contains a small amount of stabilizer.
 7. A method according to claim 6, wherein said stabilizer is a substance selected from the group consisting of trioctadecyl phosphite and dilauryl thiodipropionate.
 8. A method according to claim 1, wherein said polypivalolactone resin contains a small amount of titanium dioxide.
 9. A method according to claim 1, wherein said molded plastic articles consist of a copolymer of polypivalolactone monomer and 25 mol percent of another lactone.
 10. A method according to claim 9, wherein said other lactone is one selected from the group consisting of Beta -propiolactone and Alpha , Alpha -diethylpropiolactone. 